Marked differences in survivorship among apple roots of different diameters

Fine roots are responsible for a substantial fraction of terrestrial net primary productivity, and a better understanding of fine root production and turnover is crucial to improving global carbon and nutrient cycling models. In most studies, roots less than 1 or 2 mm in diameter ("fine roots") have been treated as structurally and physiologically identical individuals. We used minirhizotron data from 16-yr-old apple (Malus domestica) trees to investigate differences in life span and life history among fine roots whose diameters differed by tenths of a millimeter. We also introduced the use of Cox proportional hazards regression models to assess the effects of multiple covariates on root mortality. Overwinter survivorship differed markedly among diameter classes in both years: 3-12% for roots <0.3 mm in diameter, 30% for roots 0.3-0.5 mm in diameter, and 55-60% for roots 0.5-1.1 mm in diameter. Diameter was also shown to be negatively related to the risk of root mortality using a proportional hazards regression (P < 0.0001 in 1994-1995, P < 0.030 in 1995-1996). Smaller diameter fine roots were more likely to be found in densely proliferated patches, while the majority of larger diameter fine roots were found alone or with a single neighboring root. Number of neighbors was shown to be positively correlated (P < 0.0001) with the risk of root mortality in 1994-1995. Pigmentation had a marginally significant (P < 0.053 in 1994-1995, P < 0.078 in 1995-1996) negative effect on the risk of root mortality. Tetrazolium staining of brown fine roots harvested in March of 1996 indicated that 90% were viable. The majority of roots which survived from October to May in both years were >0.5 mm in diameter and brown, and these roots gave rise to new white laterals during the spring root flush. Based on the considerable differences in morphology and life history within the class of roots <1 mm in diameter, we advocate the use of a functional definition for the fine root whenever possible.